1. Field of the Invention
The present invention relates to resistors, such as thermistors, and a resistor manufacturing method used to prevent an overcurrent from flowing when a motor is overloaded.
2. Description of the Related Art
For example, in drive motors for power window devices of automobiles, positive temperature coefficient (PTC) thermistors are used to prevent an overcurrent from flowing when a motor is overloaded.
This type of thermistor has a three-layered laminated structure including a resistor material containing carbon in a resin material interposed between two conductive metal plates such as copper-plated iron plates or brass plates. Furthermore, the thermistor is installed in a motor, and electrical connection members such as lead wires are connected to the above described metal plates.
Such a thermistor has a laminated structure formed by interposing a resistor material between metal plates. Since electrical resistance increases as temperature rise, a pressure or a large current cannot be applied to the structure in the intact laminated state (or laminated shape). In other words, in a case in which electrical connection members such as lead wires are to be connected to the metal plates as described above, the presence of a resistor material prevents the connection of electrical connection members such as lead wires to the metal plates using spot or projection welding or the like while interposing the metal plates between electrodes. In addition, if connection is effected using these methods, pressure or heating deforms or melts the resin used as the binder of resistor material, and resistor characteristics are adversly affected. Furthermore, if lead wires or the like are connected to metal plates by soldering, adhesion strength becomes insufficient and endurance and reliability deteriorate.
FIGS. 15 and 16 show a conventional thermistor 110 of this type, having one pair of wire bound portions 112, 114 to which a terminal 22 and a brush pigtail 24 are connected and disposed in metal plates 116, 118 as shown. The wire bound portions 112, 114 are formed to project out from a main body portion 122 formed by the metal plates 116, 118 and a resistor material 120. Thereby, the brush pigtail 24 and the like are connected to the metal plates 116, 118 using spot or projection welding. As a result, the brush pigtail 24 and the like are connected to metal plates 116, 118 without adversely affecting resistance characteristics, and endurance and reliability can be assured.
The conventional thermistor 110 has a configuration in which the wire bound portions 112, 114 project out from the main body portion 122 including the metal plates 116, 118 and the resistor material 120, so drawbacks arise that involve numerous limitations on the direction in which the brush pigtail 24 and the like are connected, making components difficult to standardize and preventing the use of common manufacturing jigs.
In other words, the wire bound portions 112, 114 are disposed to project out from the main body portion 122, consequently reducing the disposition space within the motor of the application subject. Many limitations are thus placed on the connection space of the pigtail 24 and the like, and the degree of freedom in the connection space is low. In the thermistor 110, the forming positions of wire bound portions 112, 114 must therefore be varied with the motor type of the application subject, its installation position, or its installation direction. Connection portions are not, for example, always formed at the upper corners of the main body portion 122 as shown in FIGS. 15 and 16, and they may have to be formed at lower corners of the main body portion or on the sides thereof. As a result, thermistors must be preset as dedicated components based on individual motors, and thermistors 110 so preset above require the wire bound portions to have different forming positions. Manufacturing jigs, as holding means (so-called pallets), must be preset as dedicated for individual thermistors 110 differing in overall shape. This results in factors adversely affecting cost and productivity.
Other preceding techniques known concerning PTCs, are PTCs improved in chip shape as described in Japanese Patent Application Laid-Open (JP-A) No. 62-254402, PTCs improved in electrode position as described in Japanese Utility Model Application Laid-Open (JP-U) No. 63-5601 and Japanese Patent Application Laid-Open (JP-A) No. 2-98113, and PTCs improved in electrode film as described in Japanese Patent Application Publication (JP-B) No. 59-24521.